Milk of Magnesia, which is readily available at most retail outlets, can be as effective a braze stop as any stop off products. A typical stop off is a mixture of metallic oxide powders and a liquid; plain Milk of Magnesia is a suspension of powdered magnesium hydroxide in water. When Milk of Magnesia is heated, the water boils away, leaving the solid magnesium hydroxide. Continued heating (to just above 600° F) releases the water from the magnesium hydroxide crystal structure, leaving plain magnesium oxide. Together these components form a suspension or slurry that can be applied to the area where you do not want braze to flow. While it’s common for manufacturers to add color to their particular suspension for marketing purposes, the results are the same.
However, using any braze-stop doesn’t mean you can over use filler metal! A proper braze joint can only be achieved by using the correct amount of filler metal. By the same token, don’t over-apply stop-off! It can’t be wiped off or removed if dipped in a solution – residues have to either be ultrasonically cleaned or machined off if the surface is going to be brazeable again. Proceed with caution and call us if you have questions.